Mechanical and thermal properties of a nanopowder talc compound produced by controlled ball milling

Abstract

A powdered compound constituted by over the 95% of talc Mg3Si4O10(OH)2 with MgCO3 and CaMg(CO3)2 as minor phases was mechanically deformed by compaction and shear to a nanosized particulate (crystallite size ~5 nm) in a specifically built planetary ball mill. The mechanical milling was conducted in a controlled thermodynamic environment (25 °C and 0.13 Pa) by using low mechanical load to minimise amorphisation of the material. Mechanical τ(e) shear analysis and thermo-structural modifications of the nanostructured talc particulate were investigated after selected milling times (0, 1, 5 and 20 h). At the very early stages of milling (1 h) layer flattening, lamination and texturing of the talc particles occurred. For prolonged milling (up to 20 h), a progressive reduction of the TOT talc stacking layer coherence, from about 20–5 nm, and an increase of (001) microstrain from about 0.6–2.2 × 10−2 nm, as a non-linear function of the treatment time, were observed. A progressive increase of the specific surface area up to 28 m2/g as a consequence of the particle size reduction took place at intermediate milling times (5 h) and reduced to about 10 m2/g at prolonged milling (20 h). Even the thermo-structural behaviour of the particulate was significantly modified. For 20-h milled talc, a severe decrease of the dehydroxylation temperature from about 900–600 °C was observed with a concomitant anticipation of the recrystallisation of talc into MgSiO3 (enstatite). The τ(e) behaviour of the compound was strongly affected by the milling treatment changing from a shear-softening regime (untreated and 1 h) to a shear-hardening one (20 h). The observed changes of talc are of great importance to understand the rheology and the thermal transformation kinetics of talc compounds and can be exploited in those industrial applications that required milling of talc, such as in the production of talc-polymers nanocomposites or in medium–high-temperature ceramic processes.